A known disc prosthesis is disclosed in U.S. Pat. No. 6,419,706. U.S. Pat. No. 6,419,706 discloses a prosthesis having an elastic core and casings attached to the core. The casings engage vertebrae and partially cover the core. Each of the casings has flaps and skirts that extend toward the opposite casing. The flaps and skirts are spaced from the core when the prosthesis is an uncompressed condition. The core engages the flaps and skirts upon maximum compression of the prosthesis.